To enhance the ignition probability of electrical discharges in gases, it is common practice to illuminate the gases with ultraviolet radiation. Lamp starting is enhanced in this manner because the energy equivalent of the ultraviolet radiation exceeds the ionization potential of many gases. The ultraviolet radiation thus causes a small amount of the gases to ionize. The resulting free electrons can be accelerated by an applied electric field to have sufficient kinetic energy to produce successive, multiplicative ionization events, resulting in a discharge avalanche and breakdown of gases needed to start a lamp.
Generally, a breakdown avalanche requires at least one free electron to begin the process. Although normal background cosmic radiation can serve the purpose of generating the required free electron, the electron production rate is too low to be of practical use in applications where available ignition voltage is limited and/or where ignition time must be less than a specified interval, resulting in long time intervals, i.e., statistical time lags. The use of ultraviolet light substantially shortens these time intervals for a given applied voltage. Alternatives to ultraviolet radiation are other types of high energy radiation such as might be obtained from a variety of radioactive materials. Such alternative approaches, however, are generally costly or pose questions of safety.
Accordingly, it is desirable to provide a starting circuit for an electrodeless HID lamp which enhances ignition of an arc discharge, without requiring the use of an ultraviolet radiator or higher-energy radiators which are costly or potentially unsafe.